There's no guarantee we'll be in a better position next season. Plus we now have Thursday-Sunday.
The reason he isn't going anywhere this season is because he's only had 1 full season of first team football and there's no reason for him to want to leave anywhere yet. Not only that, there's no reason for Dortmund to be willing to sell him no matter the price (unless we're talking 150m+, which is crazy). I highly doubt it's anything to do with our previous season. Sancho just broke through this season, had a great season but they failed to win the title. He'll want to stay another season where his value won't decrease at all even if he isn't quite as good (from Dortmunds perspective), another season of development as a first team player and then he'll likely be ok with leaving. He's only been there for 2 seasons, only 1 being a first teamer, so after 3 total I can see him being ready to leave and Dortmund being more willing to negotiate.
As for his destination... There has been plenty of talk that he only wants the premier league and wants to go back to England (he's English, makes sense). That rules out Barca/Madrid/Bayern/PSG/Juve/etc. From England, Chelsea are transfer banned the next 2 windows, and just sold Hazard, but signed Pulisic and are going big to keep Hudson-Odoi. That's likely their 2 wingers (with Willian/Pedro still there) for the near future (and anyway, with Hazard gone, they're going to struggle). They have other problems to go big on as well. Arsenal never have enough money for a purchase like Sancho, nor are they a bigger draw than us. Same with Spurs, plus Pochettino likely isn't staying there super long term. He left City and Pep previously ruled out going back for him because he left in the first place, and I don't see that being a Pogba to United situation (though if Sane leaves this summer, anything is possible I guess). Still though, he'll want to be a key player/automatic starter wherever he goes, which he won't be ahead of City's attacking options, nor would he be at Liverpool ahead of Mane/Salah.
Things can change depending on player sales, but assuming Liverpools front 3 doesn't change, and there is nothing drastic at City, I'd say United are the overwhelming favorites for whenever he decides he's ready to move back to England. We desperately need a right winger, he'll be given the #7 instantly and be an instant star here, he'll be at the biggest English club around and even if we haven't been amazing in recent years, still are among the biggest clubs in the world and carry that weight to lure players (and can pay handsomely). Plus he has plenty of good mates at United which is always a plus (genuinely feel a big part of why De Gea stayed so long at United is because of Herrera and Mata).
So honestly, it's not really that important how we did this season or if we don't challenge for the title next year. Players like Sancho and De Ligt will be players who trust in their own ability and will appreciate the responsibility they would get at United of being the main players, and are players who wouldn't care much about the previous few years without them, but would trust that at an ambitious club, they could lead them into glory days again. Not saying we'll get De Ligt or for sure Sancho, but that's my view on whenever anyone says shit about United's current state being a huge black mark on us. Professional footballers, especially future stars like them, will see themselves as the people who can turn that around.